1. Field of the Invention
The present invention relates to a electrostatic image developing toner for dry-process developing a latent electrostatic image formed in an electrophotographic method, an electrostatic recording method and an electrostatic printing method.
2. Description of the Related Art
Dry-process developing devices using a powdery developing agent have widely been employed in image forming apparatuses such as electronic copiers, printers and facsimiles, in which a latent electrostatic image formed on a latent image bearing member is visualized with a developer to obtain a recorded image.
In recent years, color image forming apparatuses using electrophotographic process have broadly been employed, and digitized images are easily available. Thus, it is required to make an image to be printed at higher definition. While studying higher resolution and gradation of an image, as an improvement of a toner which visualizes a latent image, it has been studied to further conglobate and minimize in particle size for forming the image at high definition. And, since in the toners produced by the pulverizing methods, their conglobation and minimization are limited, so-called polymerized toners produced by a suspension polymerization method, an emulsification polymerization method and a dispersion polymerization method capable of conglobtaining and minimizing in particle size have been being employed.
In order to improve fluidity and chargeability of toner, a method has been proposed in which toner particles are mixed with inorganic powder such as metal oxides. Such inorganic powder is called external additives.
Examples of the inorganic powder known include silicon dioxide (i.e., silica), titanium dioxide (i.e., titania), aluminum oxide, zinc oxide, magnesium oxide, cerium oxide, iron oxide, copper oxide and tin oxide.
In order to modify hydrophobicity and chargeability of a surface of the inorganic powder, various methods have been proposed. In particular, a method is suitably used in which silica or titanium oxide powder is reacted with organic silicon compounds such as dimethyldichlorosilane, hexamethylenedisilazane and silicone oils to substitute silanol groups on surfaces of fine silica particles with organic groups for imparting hydrophobicity thereto.
In the production method of polymerized toners, toner materials having relatively low resistance are localized in the vicinity of the surfaces of toner core particles. Thus, the formed polymerized toners have low chargeability to cause background smear. In addition, the polymerized toner has a small particle diameter and thus has increased adhesive force to members, thereby raising problems such as filming and a drop in transfer efficiency. Furthermore, the polymerized toner is highly spherical to cause cleaning failure.
In view of this, attempts have been made to modify the surfaces of toner core particles to solve the aforementioned problems. The method for surface modification is, for example, dry methods in which fine particles are made to adhere onto the toner surfaces by the action of mechanical impact, and wet methods in which a resin dispersing agent is added to a dispersion liquid containing toner particles dispersed in a solvent, wherein the resin of the resin dispersing agent is different from the resin forming the toner particles. Regarding the dry methods, Japanese Patent (JP-B) No. 2838410 or other literatures disclose a toner including base particles and fine particles embedded in the surfaces thereof, wherein the toner is produced by adding the fine particles to the base particles heated to a temperature near their softening point, followed by stirring and mixing. Also, JP-B No. 2750853 discloses a toner including fine resin particles and core particles which are covered with the fine resin particles by the action of mechanical impact. In these dry methods, the fine particles are ununiform and thus cannot be attached on the toner surfaces sufficiently. As a result, the fine particles are exfoliated to cause problems such as filming and adhesion.
Regarding the wet methods, Japanese Patent Application Laid-Open (JP-A) No. 2008-090256 or other literatures disclose a method in which the surfaces of toner core particles formed of first resin particles and a colorant are partially or totally covered with second resin particles. However, according to this method, the toner core particles are covered with the second resin particles so sparsely and ununiformly that background smear and storage stability cannot be sufficiently improved, although cleanability is improved. In addition, degradation of transferability occurs.
JP-A No. 2008-233430 or other literatures disclose a toner including toner core particles and convex portions with an average diameter of 100 nm to 500 nm which are provided on the surfaces of the toner core particles, wherein the toner core particles are covered with the convex portions at a coverage rate of 10% to 80%. However, according to the production method described in Examples, the protrusions of the toner are not uniform in size, and thus the toner cannot solve problems such as background smear. The binder resin forming the convex portions has high polarity to greatly change depending on the environment and thus, is insufficient in improvement of heat resistance storage stability.
JP-A No. 2003-202701 or other literatures disclose a method in which fine resin particles are added in advance to an aqueous phase for fusion to control the particle diameter. However, in this method, the fine resin particles are incorporated into toner core particles, and as a result, the toner core particles cannot be covered with the fine resin particles in such an amount that heat resistance storage stability is improved.
According to JP-A No. 09-258480, cores are totally covered with shell layers, leading to considerable degradation of fixing property.
Some attempts to solve these problems have been made by appropriately selecting external additives. JP-A Nos. 2008-241927 and 2006-259705, JP-B Nos. 2876898, 4076681 and 3495894 and JP-A No. 2006-208493 disclose a method using an external additive having a large particle diameter. JP-A No. 09-114129, JP-B No. 2895837, and JP-A Nos. 62-17751, 05-61245, 2004-341537 and 2009-210977 disclose a method using an external additive having an opposite polarity to that of toner core particles.
The large-particle-diameter external additive is known to have a spacer effect of preventing toner core particles from being in direct contact with members. However, the above problems often arise with change in toner over time. Such a spacer effect cannot be obtained over a long period of time. This is likely because external additives are released or embedded; especially, those having a larger particle diameter are easier to be released.
In order to suppress release of external additives by the action of electrostatic force, for example, there are used external additives each having an opposite polarity to that of toner core particles or two different external additives having different polarities. In order to retain external additives, it is important to increase the contact area between external additives and toner core particles. Simply using an external additive having an opposite polarity cannot provide long-term effects.